1. Field of the Invention
The present invention pertains generally to telecommunications, and particularly to a High Speed Downlink Packet Access (HSDPA) system such as that operated (for example) in a Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN).
2. Related Art and Other Considerations
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. As wireless Internet services have become popular, various services require higher data rates and higher capacity. Although UMTS has been designed to support multi-media wireless services, the maximum data rate is not enough to satisfy the required quality of services. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
One result of the forum's work is the High Speed Downlink Packet Access (HSDPA). The HSDPA system is provides, e.g., a maximum data rate of 10 Mbps and to improve the radio capacity in the downlink. FIG. 4 illustrates a high-speed shared channel concept where multiple users 1, 2, and 3 provide data to a high speed channel (HSC) controller that functions as a high speed scheduler by multiplexing user information for transmission over the entire HS-DSCH bandwidth in time-multiplexed intervals (called transmission time intervals (TTI)). For example, during the first time interval shown in FIG. 4, user 3 transmits over the HS-DSCH and may use all of the bandwidth allotted to the HS-DSCH. During the next time interval, user 1 transmits over the HS-DSCH, the next time interval user 2 transmits, the next time interval user 1 transmits, and so forth.
HSDPA achieves higher data speeds by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller. Those responsibilities include one or more of the following (each briefly described below): shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
In shared channel transmission, radio resources, like spreading code space and transmission power in the case of CDMA-based transmission, are shared between users using time multiplexing. A high speed-downlink shared channel is one example of shared channel transmission. One significant benefit of shared channel transmission is more efficient utilization of available code resources as compared to dedicated channels. Higher data rates may also be attained using higher order modulation, which is more bandwidth efficient than lower order modulation, when channel conditions are favorable.
Radio channel conditions experienced on different communication links typically vary significantly, both in time and between different positions in the cell. In traditional CDMA systems, power control compensates for differences in variations in instantaneous radio channel conditions. With this type of power control, a larger part of the total available cell power may be allocated to communication links with bad channel conditions to ensure quality of service to all communication links. But radio resources are more efficiently utilized when allocated to communication links with good channel conditions. For services that do not require a specific data rate, such as many best effort services, rate control or adjustment can be used to ensure there is sufficient energy received per information bit for all communication links as an alternative to power control. By adjusting the channel coding rate and/or adjusting the modulation scheme, the data rate can be adjusted to compensate for variations and differences in instantaneous channel conditions.
For maximum cell throughput, radio resources may be scheduled to the communication link having the best instantaneous channel condition. Rapid channel dependent scheduling performed at the bases station allows for very high data rates at each scheduling instance and thus maximizes overall system throughput. Hybrid ARQ with soft combining increases the effective received signal-to-interference ratio for each transmission and thus increases the probability for correct decoding of retransmissions compared to conventional ARQ. Greater efficiency in ARQ increases the effective throughput over a shared channel.
With HSDPA, the physical layer becomes more complex as an additional MAC protocol is introduced: the MAC-hs. On the network side, the MAC-hs protocol is implemented in the radio base station (RBS). The MAC-hs protocol contains the retransmission protocol, link adaptation, and channel dependent scheduling. The complexity increase with HSDPA is thus mainly related to the introduction of an intelligent Layer 2 protocol in the radio base station (RBS).
In the downlink from the radio access network to the mobile station, the radio transmission is not perfectly orthogonal. As a result, when a radio signal is transmitted, a self-interference will be created. For example, when a base station transmits X amount of power in the downlink for a connection, only Y % of the power will be useful energy, with the remainder (100−Y) % creating interference for the connection.
Radio access networks typically employ a parameter such as Channel Quality Indicator (CQI) to describe radio conditions in a cell. The CQI is a measure of the quality of the common pilot CPICH as reported by each mobile station (e.g., each user equipment unit (“UE”)). Assume a scenario in which the CQI is estimated during a time of low system load (e.g., low total downlink cell power), so consequently the interference for that cell would also be very low during the time of low system load. But if system load were suddenly to increase for this scenario, as could occur when data is transmitted to the mobile station on a high-speed downlink shared channel (HS-DSCH), the high-speed downlink shared channel (HS-DSCH) would likely take all remaining cell power. In other words, the cell would then transmit at maximum downlink cell power. With the cell transmitting at maximum downlink cell power, the self-interference created the radio conditions would become worse, and certainly worse than what is represented by the previously reported “low load” CQI.
Thus, in the scenario discussed above, the input CQI for the scenario represented a better radio situation than what becomes the case later in the scenario when the high-speed downlink shared channel (HS-DSCH) is transmitting. In accordance with conventional practice, a transport format is selected for each user's TTI in the high-speed downlink shared channel (HS-DSCH) based on the user's respective CQI. The transport format affects such things as the energy per user data bit to be utilized for the transmission. When the earlier-established CQI inaccurately reflects the subsequent condition, the transport format selected for the transmission of the high-speed downlink shared channel (HS-DSCH) will be such as to use less energy per user data bit than optimal to achieve sufficient SIR, since the interference level is increased relative the CQI estimation period. Such inaccurate format selection and attendant inaccurate energy allocation typically results in an increase in failed transmissions. Failed transmission usually entails a need for retransmission. Unfortunately, retransmission results in decreased end-user throughput and negatively affects cell capacity.
What is needed, therefore, and an object herein provided for, are means, methods, and techniques for effectively establishing a high-speed downlink shared channel (HS-DSCH) that will not lead to deteriorated radio conditions.